In physics class, the girl to boy ratio is $5$ to $9$. If there are $45$ boys, how many girls are there?
Answer: A ratio of $5$ girls to $9$ boys means that there is a group of $5$ girls for every group of $9$ boys. If there are $45$ boys, then there are $5$ groups of $9$ boys each. Because we know the ratio is $5:9$ , there must be $5$ groups of $5$ girls each. There is a total of $25$ girls in physics class.